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Due to water uptake, the polymer membrane of polymer electrolyte membrane water electrolyzers (PEMWE) swells, increases in thickness and so induces swelling pressure. The water content in the membrane and catalyst coated membrane (CCM), respectively, defines the protonic conductivity, which has a significant impact on the performance of a PEMWE. In order to ensure the gas tightness of the PEMWE and increase the thermal and electrical connectivity between the different layers, the entire electrolyzer is compressed. Whether and in which way the swelling of the CCM is influenced by applying pressure or influences the surrounding layers is investigated in this study using a special compression device and X-ray computer tomography (CT). CT scans were carried out and the resulting cross-sectional images analyzed. Five different compression pressures between 0.36 and 1.63 MPa were applied for the dry (28 °C, atmospheric humidity) and wet (28 °C, surrounded by liquid water) states. The thickness change of the CCM and adjacent porous transport layers (PTL) was then measured. Due to the compression pressure, the thickness of the CCM decreased by 5%. For lower pressures, the carbon paper PTL compensates more of the swelling than the titanium felt PTL. For higher pressures, the ratio is inverse. [Display omitted] • Visualization of membrane-swelling • Increasing the clamping pressure locally reduces the water uptake of a Nafion® 212 membrane • Membrane swelling reduces the porosity of the PTL by one percentage-point. [ABSTRACT FROM AUTHOR]